/*
*	ID : Pallab
* 	PROG : A
* 	LANG : C++
* 	2012-12-09-16.04.14 Sunday
*
* 	"I have not failed, I have just found 10000 ways that won't work.
*/
#include <iostream>
#include <algorithm>
#include <vector>
#include <sstream>
#include <fstream>
#include <string>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <numeric>
#include <stack>
#include <functional>
#include <bitset>
#include <iomanip>
#include <unordered_set>
#include <unordered_map>

#include <ctime>
#include <cassert>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <climits>
#include <cstring>
#include <cstdlib>

using namespace std;

#define foR(i1,st,ed) for(int i1 = st , j1 = ed ; i1 < j1 ; ++i1 )
#define fo(i1,ed) foR( i1 , 0 , ed )
#define foE(i1,st,ed) foR( i1, st, ed+1 )
#define foit(i, x) for (typeof((x).begin()) i = (x).begin(); i != (x).end(); i++)
#define bip system("pause")
#define Int long long
#define pb push_back
#define SZ(X) (int)(X).size()
#define LN(X) (int)(X).length()
#define mk make_pair
#define SET( ARRAY , VALUE ) memset( ARRAY , VALUE , sizeof(ARRAY) )
#define line puts("")

inline void wait(double seconds) {
    double endtime = clock() + (seconds * CLOCKS_PER_SEC);
    while (clock() < endtime) {
        ;
    }
}
template<class T>
inline T fastIn() {
    register char c=0;
    register T a=0;
    bool neg=false;
    while(c<33)c=getchar();
    while(c>33) {
        if(c=='-') {
            neg=true;
        } else {
            a= (a*10)+(c-'0');
        }
        c=getchar();
    }
    return neg?-a:a;
}
const int MX=5005;
int num_count;
vector< int > adj[MX];
unordered_set<int> numbers;

inline void read() {
    num_count=fastIn<int>();
    int upto=num_count*2,tmp;
    fo(i,upto){
        tmp=fastIn<int>();
        adj[ tmp ].pb(i+1);
        numbers.insert( tmp );
    }
}
struct node{
        int a,b;
        inline void print(){
            cout<<a<<' '<<b<<'\n';
        }
};
inline void proc() {
    vector< node > result;
    for( auto &x:numbers ){
        vector<int> &all = adj[x];
        int cnt=SZ(all);
        if( cnt%2 ){
            puts("-1");return ;
        }else{
            for(int i=0;i<cnt;i+=2){
                result.pb( node{all[i],all[i+1]} );
            }
        }
    }
    for(auto &x:result)
        x.print();

}
int main() {
    int kase = 1;
#if defined( xerxes_pc )
    if (!freopen("in1", "r", stdin))
        puts("error opening file in "), assert(0);
    kase = 2;
#endif
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    fo(i,kase) {
        read();
        proc();
    }
    return 0;
}
